2019A&A...630A..73M

Context. The giant molecular cloud Sagittarius B2 (hereafter Sgr B2) is the most massive region with ongoing high-mass star formation in the Galaxy. In the southern region of the 40-pc large envelope of Sgr B2, we encounter the Sgr B2(DS) region, which hosts more than 60 high-mass protostellar cores distributed in an arc shape around an extended HII region. Hints of non-thermal emission have been found in the HII region associated with Sgr B2(DS).
Aims. We seek to characterize the spatial structure and the spectral energy distribution of the radio continuum emission in Sgr B2(DS). We aim to disentangle the contribution from the thermal and non-thermal radiation, as well as to study the origin of the non-thermal radiation.
Methods. We used the Very Large Array in its CnB and D configurations, and in the frequency bands C (4-8GHz) and X (8-12GHz) to observe the whole Sgr B2 complex. Continuum and radio recombination line maps are obtained.
Results. We detect radio continuum emission in Sgr B2(DS) in a bubble-shaped structure. From 4 to 12GHz, we derive a spectral index between -1.2 and -0.4, indicating the presence of non-thermal emission. We decomposed the contribution from thermal and non-thermal emission, and find that the thermal component is clumpy and more concentrated, while the non-thermal component is more extended and diffuse. The radio recombination lines in the region are found to be not in local thermodynamic equilibrium but stimulated by the non-thermal emission.
Conclusions. Sgr B2(DS) shows a mixture of thermal and non-thermal emission at radio wavelengths. The thermal free-free emission is likely tracing an HII region ionized by an O 7 star, while the non-thermal emission can be generated by relativistic electrons created through first-order Fermi acceleration. We have developed a simple model of the Sgr B2(DS) region and found that first-order Fermi acceleration can reproduce the observed flux density and spectral index.